Snap action switch devices



y 1966 c. G. HEMMENS 3,259,709

SNAP ACTION SWITCH DEVICES Filed July 16, 1964 \vw ENTOR Cecm GODFREYHmqMENs BYWMpM United States Patent M 3,259,709 SNAP ACTION SWITCHDEVICES Cecil Godfrey Hemmens, Purton, England, assignor to Plessey-UKLimited, Essex, England, a British comany P Filed July 16, 1964, Ser.No. 383,174 Claims priority, application Great Britain, July 24, 1963,29,345 63 17 Claims. (Cl. 200-65) This invention relates to snap actionswitch mechanisms and, more particularly, to a switch adapted toenergize and de-energize an electrical circuit control device, e.g. apotentiometer, in response to control movement of the control device. 7

Accordingly, a general object of this inventionis to provide a snapaction switch assembly which energizes and de-energizes an electricalcontrol device in response to control movement of the device and, in itsmore specific aspects, to achieve such operation while maintaining aswitch construction which is simple, economical and readily assembled.

For the achievement of these and other objects, this inventioncontemplates a switch mechanism having a conductive contact springextending between first and second stationary contacts which aresuitably connected in the circuit of an electrical circuit controldevice. The contact spring is suitably anchored at the firs-t contactand has a free end biased into engagement with and free for movementwith respect to the second contact. The contact spring is so mounted asto be capable of flexing on itself to vary its extension between thestationary contacts as well as being movable into and out of engagementwith the second contact. To achieve movement of the contact spring withrespect to the second contact, and also flexing of the spring on itself,the free end of the spring is positioned in the path of a movable cammember. Upon engagement of the cam member with the spring and subsequentrelative movement therebetween, the spring is flexed inwardly toward thefirst contact. The cam eventually releases the spring for movementtoward its unflexed position and the spring is simultaneously cammedaway from the second contact to open the switch circuit. A suitable stopmember is provided to limit the relative movement of the cam in onedirection with respect to the contact spring and the position of thestop member with respect to the first con-tact and the contact spring issuch that the spring is held out of engagement with the first contact tohold the switch circuit open after cam movement has been interrupted.

Other objects and advantages will be pointed out in, or be apparentfrom, the description and claims, as will obvious modifications of theembodiments shown in the drawing, in which:

FIG. 1 is an axial view illustrating the switch in closed circuitposition with the contact spring partially flexed;

FIG. 2 is a similar view illustrating the switch just prior to releaseof the contact spring;

FIG. 3 is a similar view illustrating the switch after release of thespring and with the switch in its open circuit position;

FIG. 4 is a similar view illustrating the switch after initial movementfrom its off position (open circuit) toward its on position (closedcircuit);

FIG. 5 is an enlarged view of the cam and the free end of the contactspring;

FIG. 6 is an axial view of an alternative embodiment illustrating theswitch in its closed circuit position prior to engagement between thecontact spring and cam;

FIG. 7 is a similar View of the alternative embodiment illustrating theswitch after partial flexing of the contact spring and while still in aclosed circuit position;

3,259,799 Patented July 5, 1966 ICE FIG. 8 is a similar view of thealternative embodiment illustrating the switch in its open circuitposition;

FIG. 9 is a similar view of the alternative embodiment illustrating theswitch during initial movement from its off position to its on position;and

FIG. 10 is an axial view of a rotor construction which can be used inthe alternative embodiment.

The switch mechanism of this invention is adapted for use with any oneof a number of electrical circuit control devices, such as apotentiometer. The details of the circuit control device are notnecessary to a proper understanding of this invention and for thatreason it has neither been illustrated in the drawing nor will it bespecifically described.

FIGS. 1-5 illustrate a switch embodying this invention and used incombination with a rim actuated control device 10 wherein actuating rim12 is supported for rotation about fixed base plate 14. Boss 16 extendsaxially from base plate 14 and is preferably arranged coaxially with rim12. Switch contacts 18 and 20 also extend axially of base plate 14 andare suitably connected in circuit with control device terminals 19 and21 to control electrical energization and de-energization of the controldevice. Contact spring 22 extends between the stationary contacts tocomplete an electrical circuit through the switch. End 24 of the contactspring is turned back on itself to loop over contact 20 and opposite end26 of the contact spring is also turned back on itself to provide arounded end for a purpose which will become apparent as this descriptionproceeds.

Preferably, contacts 18 and 20 are positioned on opposite sides of boss16 with the circumference of the boss extending to a point ofisetlaterally from an axially extending plane including contacts 18 and 20or, in other words, offset from a plane including the contacts andextending normal to the base plate or the plane of the drawing. Contactspring 22 is assembled by fitting end 24 over contact 20 and flexing thecontact spring around boss 16, this flexing causes the spring to bowintermediate its ends and produces a biasing force urging the contactspring into engagement with both contacts 18 and 20 thereby maintainingelectrical engagement therebetween and without unduly high contactresistance.

With contact spring 22 so mounted, its end 26 can be moved clockwiseabout boss 16 to open the electrical circuit and, being flexedintermediate its ends, the contact spring will readily flex inwardlytoward contact 20 in response to a force generally parallel to itsextension between the stationary contacts or in a radial direction inthe illustrated embodiment. To move spring 22 from its closed circuitposition of FIG. 1 to its open circuit position of FIG. 3, rim 12 isprovided with cam 28 which projects radially inwardly toward boss 16 andthe contact spring with end 26 of the contact spring positioned in thepath of rotation of the cam. Stop 38, preferably fixed on base plate 14,is also positioned in the path of movement of the cam to determine theoil position of the switch as will be described more completelyhereinafter.

During the major portion of rim movement, cam 28 is out of engagementwith spring end 26 so that the contact spring bridges between contacts18 and 20 and the switch circuit is closed. In moving in acounterclockwise direction, preferably from a maximum-voltage positionto a minimum-voltage position where control device 10 is apotentiometer, cam 28 will move into engagement with end 26 of thecontact spring. Cam 28 includes camming surface 30 extending angularlyinwardly from leading edge 32 and falls oif sharply at trailing edge 34with the transistion from camming surface 30 to trailingedge 34 beingthrough radius 36. Trailing edge 34 and radius 36 provide a cammingsurface which extends angularly from camming surface 30 toward the rim.As rim 12 is rotated counterclockwise or in the direction of the arrowin FIGS.

1-3, leading edge 32 engages contact spring end 26 and continued cammovement flexes the contact spring on itself and about boss 16, however,due to the angle of surface 30 (the inward radial extension thereofincreases from the leading to the trailing edge) the tendency is to holdend 26 in engagement with contact 18. Inward flexing of the contactspring continues until end 26 of the contact spring is transferred fromsurface 30 to radius 36 whereupon interaction between arcuate surface 27of the spring end and radius 36, coupled with the energy stored in thecontact spring due to its having been flexed, snaps end 26 toward theinner rim periphery and into engage ment with trailing edge 34.

Stop member 38 is arranged for engagement by cam leading edge 32 toterminate rim movement in a counterclockwise direction and to determinethe off position for the control device. Cam 28 is preferablyconstructed so thart the angular distance between its leading edge andtrailing edge is greater than the angular distance between stop 38 andthe point of engagement between contact spring end 26 and contact 18 sothat when the cam is in engagement with stop 28 contact spring end 26 isheld away from contact 18 to thereby maintain the switch in an opencircuit condition.

With these structural details in mind, the operation of the switchdevice will now be described. Assuming rim 12 to be initially in aposition wherein cam 28 is spaced from the contact spring, e.g. thedotted line showing of cam 28 in FIG. 1, the control device can beadjusted without affecting the closed circuit position of the switch. Asrim 12 is rotated counterclockwise toward the off position determined bystop 38, carn leading edge 32 engages free end 26 of the contact springand, since the spring cannot rotate with cam 28, end 26 is movedradially inwardly toward contact 20 causing the contact spring to fiexon itself in the direction of its extension between contacts 18 and 20'.Continued movement of cam surface 30 with respect to the contact spring(FIG. 1) increases the inward flexing of the contact spring until springend 26 reaches radius 36, see FIG. 2. As surface 27 of the contactspring end engages radius 36 the biasing force stored in the contactspring during the inward flexing, together with the interaction ofradius 36 and surface 27,. causes surface 27 to move over radius 36freeing the spring for movement toward its unflexed position while alsocamming spring end 26 in a clockwise direction with a snap action awayfrom contact 18 to establish the open circuit condition of the switchwith end 26 in engagement with trailing edge 34 as illustrated in FIG.3. When cam leading edge 32 strikes stop 38, illustrated in FIG. 3, thecontact spring remains in engagement with trailing edge 34 and, sincethe arcuate extension between the leading and trailing edge is greaterthan the arcuate distance between the stop and the point of engagementbetween the contact and the spring, the contact spring is held away fromcontact 18 to maintain an open circuit position. In effect cam 28includes two camming surfaces, surface 30 the radial inward extension ofwhich increases from the leading to trailing edge so as to slope towardthe contacts and contact spring and that formed by surfaces 34 and 36which slopes away from the contacts and contact spring. In theillustrated embodiment these surfaces are oppositely angled with respectto a radial line from the axis of rim rotation and this in combinationwith the configuration of the spring end produces the necessary cammingaction to achieve desired switch opening. Preferably spring end 26 has agenerally arcuate surface so as to present an angular face to radius 36to cam the spring end in a clockwise direction. It will be appreciatedthat the engagement of the radius and spring end occurs prior to the camengaging stop 38 so that the spring end follows the cam for a smallangular distance but an amount insuflicient to retzlose the contactspringon contact 18. Other configurations can be used to cam the contactspring away from contact 18, e.g. end 26 could be V-shaped, it couldhave one angular surface which engages the cam trailing edge before thecam reaches the stop or, conceivably, contact spring end 26 could beflat and merely the trailing edged surface angled to rely on theinteraction of the angled surface and stored spring force to achieve thecamming action.

Should it now be desired to switch the control device from its offcondition of FIG. 3 to its on condition, rim 12 is rotated clockwise.During initial clockwise rotation from the position of FIG. 3, springend 26 moves with cam 28 away from contact pin 18 and also decreases theradial extension of the contact spring toward the rim (see FIG. 4). Uponcontinued clockwise rotation of the rim, spring end 26 eventually passesover radius 36 freeing the spring for return movement of the spring intoengagement with contact 18. This return movement of the spring intoengagement with contact 18 is influenced by the initial resiliency inthe spring due to flexing over boss 16 and also the stress created inthe spring during its rotation away'from stop 38. Re-engagement withcontact 18 restores the closed circuit position with a snap action andenergizes the control device. During further movement of rim 12, theclosed circuit condition remains undisturbed until the rim is againrotated toward engagement with stop 38 whereupon the control device isturned off in the manner described above.

FIGS. 6-10 illustrate an alternative embodiment of this invention whichillustrates that the invention is not limited to use in rim actuatedcontrol devices nor to the particular cam configuration of FIGS. 1-5.The construction and arrangement of the stationary contacts, the contactspring and boss are identical to that of the switch arrangementdescribed and illustrated in FIGS. 1-5 in that contact spring 22a hasone end 24a looped on contact 20a and has its free end 26a turned backon itself and engaging contact 18a under'the influence of the biasingforce built up in the con-tact spring as a result of being bowed aroundboss 16a which, in this embodiment, includes an axial bore 39 forreceipt of rotatable control shaft 40. In this embodiment, contactspring 22a is partially disposed in a groove defined between C-shapedridge 41 and boss 41a and actuating rotor 42 carries cam 44. The rotoris suitably connected for rotation with shaft 40 to move cam 44 into andout of engagement with end 26a of the contact spring to effect switchoperation in a manner similar to that described above in relation toFIGS. 15

More particularly, cam 44- includes leading edge 46 and camming surface48 which extends from the leading edge generally transversely of thedirection of extension of contact spring 22a between the stationarycontacts. In this instance, the extension of the contact spring is in agenerally radial direction and camming surface 48, similar to cammingsurface 30, is sloped so that its radial inward extension increases fromits leading edge to its trailing edge 50. Trailing edge 58 is radiallyaligned with the axis of rotation but, if desired, could be disposed atan angle to a radial line in the same directions as or oppositely fromcamming surface 48. The arcuate distance between stop 52 and the pointof contact between end 26a and contact 18a is less than the arcuateextension between the leading and trailing edges of cam 48 so that whenthe cam engages the stop the contact spring is held out of engagementwith upper contact 13a.

The operation of this embodiment is identical to that described above inthat as shaft 40 is rotated counterclockwise cam 44 moves towardengagement with contact spring end 26a. The contact spring is initiallyengaged by leading edge 46 and is transferred onto cam surface 48whereupon the contact spring flexes inwardly on itself with furthermovement of t-he cam. Inward flexing continues and end 26 is eventuallytransferred from surface 48 to trailing edge 50 whereupon, under theinfluence of its own resiliency, it is snapped into engagement with thetrailing edge and is moved out or engagement with contact 18a. With thisarrangement stop 52 can be engaged by cam leading edge 46 or, ifdesired, by edge 54 of the cam carrying rotor extension 56. In

7 either instance, the open circuit condition of the switch ismaintained by the angular extension between stop 52 and cam trailingedge 50 being greater than the angular extension between the stop andcontact 18a.

I Although this invention has been illustrated and described inconnection with embodiments achieving control on the basis of rotatingmovement it will be appreciated that it could be used equally as well incontrols using other types of movement, such as linear control movement.Therefore, it will be apparent to those skilled in the art that this andvarious other changes and modifications may be made Without departingfrom the spirit of the invention or from the scope of the appendedclaims.

What I claim is:

- 1. A switch for use in combination with rotary electrical controldevices having a rotatable control member, said switch comprising, incombination,

a circular base plate,

first and second fixed contacts relatively spaced on said base plate,

a fixed boss extending axially from said base plate and positionedintermediate said first and second contacts, said boss including aportion disposed laterally of an axially extending plane passing throughsaid first and second contacts,

a resilient electrically conductive contact spring having opposite ends,one end of 'said contact spring anchored to said first contact, theopposite end releasably engaging said second contact and said contactspring engaging the laterally disposed portion of said boss intermediatesaid ends, said contact spring being fiexed intermediate its ends byengagement with said boss and biased into engagement with said secondcontact,

cam means connected to and rotatable with said control member relativeto said contacts and contact w said opposite end of said contact springextending into the path of movement of said cam means and said cam meanscharacterized by having a first surface of predetermined angularextension projecting to- 'ward said contacts to flex said contact springtoward said first contact upon engagement therewith, said cam means alsoincluding a trailing edge surface disposed at an angle to said firstsurface and extending away from saidcontacts permitting expansion ofsaid contact spring and cooperating to cam. said opposite contact springend away from said second contact,

and stop means fixed on said base plate and disposed in the path ofmovement of said cam means to limit movement of said cam means relativeto said contacts, said stop means angularly spaced from said secondcontact a distance less than the angular extension of said cam means andsaid cam means trailing edge holding said contact spring out ofengagement with said second contact when said cam means is in engagementwith said. stop member.

2. The switch of claim 1 wherein said contact pins are spaced apart in aradial direction on said base plate and said boss is generallycylindrical and is intersected by said axially extending plane.

3. The switch of claim 2 wherein said contact spring ends engage saidcontact pins to one side of said plane and said contact spring engagessaid boss on an opposite side thereof so thatsaid contact spring isurged into engagement with said contacts by the resiliency of saidcontact spring. P

4. The switch of claim 3 wherein said cam means includes a leading edgeand wherein said first surface has an increasing inward radialprojection from said leading edge to said trailing edge.

5. The switch of claim 4 wherein said opposite spring end has agenerally arcuate configuration.

6. The switch of claim 5 wherein said trailing edge extends from saidfirst surface in a direction away from said contacts and is oppositelyangled with respect to a radial line and as compared to said firstsurface.

7. The switch of claim 4 wherein said rotatable control member includesa rim rotatable about the periphery of said base plate and wherein saidcam means is carried on said rim and projects inwardly thereof.

8. A snap action switch for use in combination with electrical controldevices having a movable control member, said switch comprising, incombination,

a base plate,

first and second fixed contacts relatively spaced on said base plate,

a boss extending laterally from said base plate and positionedintermediate said first and second contacts, said boss including aportion disposed laterally of. a plane extending laterally of said baseplate and including said first and second contacts,

a resilient electrically conductive contact spring having opposite ends,one end of said contact spring anchored to said first contact, theopposite end releasably engaging said second contact, and said contactspring engaging the laterally disposed portion of said boss intermediatesaid opposite ends, said contact spring being flexed intermediate itsends by engagement with said boss and biased into engagement with saidsecond contact,

cam means connected to and movable with said control member relative tosaid contacts and contact spring,

J said opposite end of said contact spring extending into the path ofmovement of said carn means and said cam means characterized by having afirst surface of predetermined extension projecting toward said contactsto flex said contact spring toward said first contact upon engagementtherewith, said cam means including a trailing edge disposed at an angleto said first surface and extending away from said contacts permittingexpansion of said contact spring and cooperating to cam said oppositecontact spring end away'from said second contact,

and stop means fixed on said base plate and disposed in the path ofmovement of said cam means to limit the movement of said cam meansrelative to said contact pins and spaced from said second contact in thedirection of movement of said cam means a distance less than saidpredetermined extension of said cam means and said cam means trailingedge holding said contact spring out of engagement with said secondcontact when said cam means is in engagement with said stop member.

9. The switch of claim 8 wherein said contact spring ends engage saidcontacts to one side of said plane and said contact spring engages saidboss on an opposite side thereof so that said contact spring is urgedinto engagement with said contacts by the resiliency of said cont-actspring.

10. The switch of claim 8 wherein said opposite spring end includes atleast one angular surface engageable with said trailing edge andcooperating therewith to cam said opposite spring end away from saidsecond contact.

1'1. The switch of claim 10 wherein said cam means said first contactspring end anchored on said first contact and said second end thereofreleasably engaging said second contact,

means defining a surface spaced laterally from a line extending betweensaid contacts and engaging said contact spring intermediate its ends,said contact spring being flexed intermediate it ends and biased towardsaid second contact,

cam means,

means supporting said cam means for movement relative'to said cont-actsand in a direction transversely of said line extending between saidcontacts, said second contact spring end disposed in the path ofmovement of said cam means and said cam means including an angular camsurface arranged transversely of said line for engagement with saidsecond contact spring end to flex said contact spring along said linetoward said first contact as said cam means moves relative to saidcontacts, said angular surface having a predetermined extension to atrailing edge and said trailing edge extending at an angle from saidangular surface and away from said contacts,

and means fixed relative to said contacts for limiting travel of saidcam means relative to said contacts and spaced from said second contacta distance less than said predetermined extension to said trailing edgeso that said contact spring is flexed toward said first contact duringengagement with said angular cam surface and expands along said trailingedge away from said second contact upon engagement with said trailingedge.

13. A switch comprising, in combination,

' first and second relatively spaced terminal means,

a resilient electrically conductive contact spring extending betweensaid terminal means and having a free end and a relatively fixed end, Ianchor means for said fixed contact spring end including said firstterminal means,

actuating means for moving said free end of said contact spring into andout of engagement with said second terminal means and including cammeans engageable with said free end, said cam means arranged to extendgenerally transversely of a line between said terminal means and'flexing said contact spring inwardly toward its fixed end as cam meansmoves relative to said contact sprin said cam means further operative topermit said contact spring to expand outwardly of its fixed end and awayfrom said second terminal means after a predetermined amount of movementof said cam means relative to said contact spring,

and means engageable with said actuating means for limiting movement ofsaid cam means relative to said contact means to stop said cam meansafter it has moved said predetermined amount and operative to :hold saidcontact spring out of engagement with said second terminal means.

14. The switch of claim 13 wherein said anchor means also includes asurface spaced laterally of a line between said terminal means andengaging said contact spring intermediate its ends to flex said contactspring and bias said free end of said contact spring into engagementwith said second terminal means.

15. The switch of claim 13 wherein said cam means includes a trailingedge extending in a direction away from said terminal means and whereinsaid free end of said contact spring has at least one angular surfaceengageable with said trailing edge to cooperate in camming said contactspring away from said second terminal means as said contact springexpands outwardly.

16. A switch for use in combination with rotary electrical controldevices having a rotatable control member, said switch comprising, incombination,

a circular base plate,

first and second fixed contacts relatively spaced on said base plate,

a boss extending axially from said base plate and positionedintermediate said first and second contacts, said boss including aportion disposed laterally of an axially extending plane passing throughsaid first and second contacts,

a resilient electrically conductive contact spring having opposite ends,one end of said contact spring anchored to said first contact, theopposite end releasably engaging said second contact and said laterallydisposed portion of said boss engaging said contact spring intermediateits ends, said con-tact spring being flexed intermediate its ends byengagement with said boss and biased into engagement with said secondcontact,

a shaft for rotating said control member,

an actuator connected to and rotatable with said shaft,

said actuator including cam means arranged for movement into and out ofengagement with said opposite end of said contact spring and supportedradially outwardly of said second contact, said cam means characterizedby having a first surface of predetermined angular extension projectingtoward said contacts to flex said contact spring toward said firstcontact upon engagement therewith, said cam means also including atrailing edge disposed at an angle to said first surface and extendingaway from said contacts permitting expansion of said contact spring andcooperating to cam said opposite contact spring end away from saidsecond contacts,

and stop means fixed on said base plate and disposed in the path ofmovement of said cam means to limit the movement of said cam meansrelative to said contacts, said stop means angularly spaced from saidsecond contact a distance less than the angular ex tension of said cammeans and said cam means trailing edge holding said contact spring outof engagement with said second contact when said actuator is inengagement with said stop member.

17. A switch comprising, in combination,

first and second relatively spaced terminal means,

a resilient electrically conductive contact spring extending betweensaid terminal means and having a free end and a relatively fixed end,

anchor means for said fixed contact spring end including said firstterminal means,

actuating means for moving said free end of said contact spring into andout of engagement with said second terminal means and including cammeans engageable with said free end, said ca-m means arranged to extendgenerally transversely of a line between said terminal means and flexingsaid contact spring inwardly toward its fixed end as cam means movesrelative to said contact spring, said cam means further operative topermit said contact spring to expand outwardly of its fixed end and awayfrom said second terminal means amount of movement of said cam meansrelative to said con-tact spring,

means engageable with said actuating means for limiting movement of saidcam means relative to said contact means to stop said cam means after ithas moved said predetermined amount and operative to hold said contactspring out of engagement with said second terminal means,

and means intermediate said contact springends and in contact therewithadapted normally flex said spring free end toward said second terminalmeans.

No references cited.

KATHLEEN H. CLAFFY, Primary Examiner.

D. SMITH, JR., Assistant Examiner.

after a predetermined

1. A SWITCH FOR USE IN COMBINATION WITH ROTARY ELECTRICAL CONTROLDEVICES HAVING A ROTATABLE CONTROL MEMBER, SAID SWITCH COMPRISING, INCOMBINATION, A CIRCULAR BASE PLATE, FIRST AND SECOND FIXED CONTACTSRELATIVELY SPACED ON SAID BASE PLATE A FIXED BOSS EXTENDING AXIALLY FROMSAID BASE PLATE AND POSITIONED INTERMEDIATE SAID FIRST AND SECONDCONTACTS, SAID BOSS INCLUDING A PORTION DISPOSED LATERALLY OF AN AXIALLYEXTENDING PLANE PASSING THROUGH SAID FIRST AND SECOND CONTACTS, ARESILIENT ELECTRICALLY CONDUCTIVE CONTACT SPRING HAVING OPPOSITE ENDS,ONE END OF SAID CONTACT SPRING ANCHORED TO SAID FIRST CONTACT, THEOPPOSITE END RELEASABLY ENGAGING SAID SECOND CONTACT AND SAID CONTACTSPRING ENGAGING THE LATERALLY DISPOSED PORTION OF SAID BOSS INTERMEDIATESAID ENDS, SAID CONTACT SPRING BEING FLEXED INTERMEDIATE ITS ENDS BYENGAGEMENT WITH SAID BOSS AND BIASED INTO ENGAGEMENT WITH SAID SECONDCONTACT, CAM MEANS CONNECTED TO AND ROTATABLE WITH SAID CONTROL MEMBERRELATIVE TO SAID CONTACTS AND CONTACT SPRING, SAID OPPOISTE END OF SAIDCONTACT SPRING EXTENDING INTO THE PATH OF MOVEMENT OF SAID CAM MEANS ANDSAID CAM MEANS CHARACTERIZED BY HAVING A FIRST SURFACE OF PREDETERMINEDANGULAR EXTENSION PROJECTING TOWARD SAID CONTACTS TO FLEX SAID CONTACTSPRING TOWARD SAID FIRST CONTACT UPON ENGAGEMENT THEREWITH, SAID CAMMEANS ALSO INCLUDING A TRAILING EDGE SURFACE DISPOSED AT AN ANGLE TOSAID FIRST SURFACE AND EXTENDING AWAY FROM SAID CONTACTS PERMITTINGEXPANSION OF SAID CONTACT SPRING AND COOPERATING TO CAM SAID OPPOSITECONTACT SPRING END AWAY FROM SAID SECOND CONTACT, AND STOP MEANS FIXEDON SAID BASE PLAT AND DISPOSED IN THE PATH OF MOVEMENT OF SAID CAM MEANSTO LIMIT MOVEMENT OF SAID CAM MEANS RELATIVE TO SAID CONTACTS, SAID STOPMEANS ANGULARLY SPACED FROM SAID SECOND CONTACT A DISTANCE LESS THAN THEANGULAR EXTENSION OF SAID CAM MEANS AND SAID CAM MEANS TRAILING ENDHOLDING SAID CONTACT SPRING OUT OF ENGAGEMENT WITH SAID SECOND CONTACTWHEN SAID CAM MEANS IS IN ENGAGEMENT WITH SAID STOP MEMBER.